Various surface coating agents using resin aqueous emulsions have been proposed as surface coating agents for vulcanized rubber-molded products.
Patent Document 1 proposes using, as a coating agent for electrophotographic device members, a material comprising, as a main component, an urethane resin emulsion obtained by chain-extending with water an NCO-terminated polyurethane prepolymer obtained from a polyolefinpolyol, diisocyanate and glycol, while dispersing the prepolymer in the water.
When the coating agent comprising the urethane resin emulsion is used as a surface coating agent for general vulcanized rubber-molded products, among various measurement items shown in Examples, described later, bleeding evaluation (70° C., 30 days), evaluation of the flexibility of the coating agent by a bending test, and evaluation of adhesion to vulcanized rubber by an adhesion test show satisfactory results; however, evaluation of adhesion to vulcanized rubber due to bloom from the rubber by a high-temperature and high-humidity test shows unsatisfactory results.
Patent Document 2 discloses a coating composition for top-coating used on an organic resin based-undercoat layer, the composition comprising:
(a) an organosilane or a hydrolyzate or condensate thereof;
(b) a vinyl-based polymer containing a silyl group or a vinyl polymer containing a silyl group and an antraviolet-stabliity group; and
(c) an organometallic compound that serves as a hydrolysis-condensation catalyst;
the composition having a specific weight ratio of component (b)/component (a).
Moreover, Patent Document 3 discloses an aqueous coating composition having excellent weather resistance, solvent resistance, etc., the composition comprising a combination of a liquid A comprising an acrylic resin emulsion with a liquid B essentially comprising a water-dispersible polyisocyanate, wherein the liquid A contains a copolymer of a hydroxyl group-containing polymerizable monomer and a cycloalkyl group-containing polymerizable monomer, and the equivalent ratio of the isocyanate groups of the liquid B to the hydroxyl groups of the liquid A is specified.
However, when the coating agent compositions of Patent Documents 2 and 3 were used as surface coating agents for general vulcanization-molded products, it was confirmed that although evaluation of adhesion to vulcanized rubber by an adhesion test showed superior results, the abovementioned other three evaluations, that is, bleeding evaluation, evaluation of the flexibility of the coating agent by a bending test, and evaluation of the reduction in adhesion to vulcanized rubber due to bloom from the rubber by a high-temperature and high-humidity test, showed inferior results.